Synesthesia is the crossing of senses, a phenomenon whereby stimulation of one sensory modality leads to automatic and involuntary experience of stimulation in a second sensory modality. Examples: to smell colors and to visualize sounds as colors.

For people with synesthesia, ordinary stimuli such as music, numbers, or words are imbued with an “extra” perceptual quality not shared by most members of the population. Music may trigger swirling shapes, numbers may have color, or words may evoke localized taste sensations felt in the mouth. For people who have synesthesia, such experiences are considered completely normal, and they can't imagine a world without them. The absence of any obvious outward manifestation of having synesthesia has contributed to its historical obscurity in scientific research. However, much of the contemporary research is interested in the cognitive and neural basis of conscious perceptual experiences, and synesthesia is back on the scientific agenda. Synesthesia also turns out to be far more common than previously believed. Recent prevalence estimates suggest that colored letters and numbers (grapheme-color synesthesia) affects 1 to 2% of the population, with some form of synesthesia affecting 1 of the 5 classical senses in 4.4% of the population. This high prevalence has led people to speculate on the evolutionary significance of synesthesia. This entry describes the key characteristics and varieties of synesthesia, neural and genetic basis of synesthesia, and some reasons why synesthesia exists.

Key Characteristics

Synesthesia has a number of core characteristics that, collectively taken, constitute a more formal definition of synesthesia that is agreed upon by most researchers in the field. First, the experiences are normally defined as being perceptual in nature. For example, a grapheme-color synesthete sees that a letter A evokes red rather than remembers “red” as a verbal memory label. Functional imaging studies are consistent with this view. However, there are some borderline examples. Not all synesthetes see their colors externally, but rather in their mind's eye. Some researchers also extend the definition of synesthesia to nonperceptual associations, such as attributing genders and personalities to graphemes (e.g., “3 is male and a jerk”). This could also be construed as a related characteristic (e.g., due to more widespread changes in the brains of synesthetes), rather than a type of synesthesia in its own right. Second, synesthesia is elicited. The synesthetic experience (known as the concurrent) is elicited by some stimulus (known as the inducer). This distinguishes synesthesia from hallucinations, in which there is typically no known inducer. As such, all types of synesthesia can be named in terms of an inducer-concurrent pair (e.g., grapheme-color, music-vision, lexical-gustatory). It is to be noted that the inducer is not always an externally perceived stimulus. Thus, thinking of an arithmetical sum (e.g., 5 + 2) can trigger a synesthetic color, and thinking of an unretrievable word in a tip-of-the-tongue state can trigger a taste. The relationship between the inducer and the concurrent is normally stable over time, and it is normally unidirectional. Seeing a letter ‘A’ may trigger red, but seeing red doesn't elicit a percept of the letter A (however, there is more convincing evidence for implicit bidirectionality in synesthesia). Finally, synesthesia is assumed to occur automatically. It cannot be switched on or off at will, and this distinguishes synesthesia from most forms of imagery. Experimentally, this has been extensively studied using a synesthetic variant of the Stroop test. In the original version of the Stroop test, people are slower at naming the ink color of a word (e.g., RED) if it is incongruently colored (e.g., in green ink) than congruently colored (i.e., red ink). In the synesthetic version of the Stroop test, synesthetes show a similar effect if the grapheme is printed in an incongruent color to their synesthesia. Interestingly, there appears to be greater interference if opponent colors are used (e.g., a synesthetically red grapheme printed in green) than non-opponent color (e.g., a synesthetically red grapheme printed in yellow), suggesting that the interference has a perceptual rather than purely mnemonic component to it.

The Varieties of Synesthesia

Although there are some common principles that are common to all (or almost all) types of synesthesia, there is a large range of different combinations of inducer-concurrent pairings. These are now briefly summarized.

Grapheme-Color Synesthesia

This variety of synesthesia has been extensively studied. Top-down influences play a large role in determining the color, such that ambiguous graphemes take on the color implied by their context (e.g., a vertical line may take on either the color of the letter I or the number 1, depending on whether it is surrounded by letters or numbers). Bottom-up influences (e.g., particular fonts) may also affect the intensity/saturation of the hue. When learning a second alphabet (e.g., Cyrillic), the colors normally migrate across based on visual and/or phonetic similarity.

Some synesthetes see the color “out there on the page” like a colored overlay (these have been termed
projectors), whereas others see it elsewhere (termed associators), often in their mind's eye. One suggestion is that projected colors are elicited earlier in the visual stream. A similar suggestion is that for some grapheme-color synesthetes, the color is elicited by “lower” levels of processing (e.g., the grapheme shape and early color processing), whereas for others it is elicited by “higher” levels (e.g., the meaning of numbers and possibly later stages of color processing).

Colored-Hearing Synesthesia

The term colored hearing was historically used as a synonym for synesthesia (farbenhören in German, audition colorée in French). In reality, the term denotes a number of distinct subtypes such as music-vision (or indeed nonmusical sounds) and speech-color, in which spoken language (but not other heard sounds) trigger color. With regard to the former, the visual experiences from music consist not only of color, but also of movement, shape, size, and location. With regard to the latter, it is typically the first letter of a spoken word that determines its color. Thus, photo and people would tend to be colored the same (because of the letter P) but photo and
fish would not (despite the shared initial phoneme). These synesthetes often perceive the spoken word spelled out like ticker tape. This suggests a two-stage process by which phonemes are converted to letter strings and the letters trigger the color. The apparent rarity of true phoneme-color synesthesia has implications for theories of synesthesia.

Synesthesia for Spatial Sequences

Ordered sequences such as days, months, numbers, and the alphabet are often perceived as arranged in a particular sequence in space. Numbers are typically arranged from left-to-right, at least in European and North American synesthetes, although they can also twist and turn through any direction of space. This, to some extent, resembles the normal spatial bias in numerical processing found in people who lack synesthesia.

Synesthesia Involving Taste and Smell

These forms of synesthesia are rarer. Richard Cytowic documented the case of MW, “the man who tasted shapes,” for whom flavors elicited tactile shapes that could be felt and explored on the hands. Synesthesia in which taste or flavors are the concurrent experience have been noted to be triggered by words that are either read or heard (e.g., New York tasting of runny egg yolk). In all cases in which words trigger tastes, it has been observed that similar sounding words tend to elicit similar tastes, suggesting that there is an underlying logic to the associations.

Synesthesia Involving Touch and Pain

Pain has been noted to induce synesthetic visual experiences. Synesthesia in which tactile experiences are concurrent were, until recently, considered to be rare, although recent research casts doubt on this. In particular, a novel variety, so-called mirror-touch synesthesia, has been documented in which observing touch to another human (but not object) triggers somatotopically appropriate sensations on the perceiver's own body. This has been demonstrated both with functional magnetic resonance imaging (fMRI) and using an interference paradigm in which felt touch must be discriminated from observed touch.

Neural and Genetic Basis

Functional Imaging and Electrophysiological Studies

A number of functional imaging studies have shown activity in human area V4, specialized for color perception, when processing speech or visual graphemes. Similarly, synesthetic tactile experiences engage the primary and secondary somatosensory areas more than controls. These studies suggest that synesthesia uses some of the same neural substrates that support veridical perception. Studies using fMRI have also directly contrasted synesthesia with color imagery and learned color associations and found that synesthesia is different from both. A recent event-related potential study of speech-color reports differences in amplitude in the EEG potential between synesthetes and controls at 122 milliseconds.

A number of other regions outside of the traditionally defined sensory cortex appear to be involved in synesthesia, particularly in the frontal and parietal regions. These may, in normal perception, be multisensory convergence zones and may be particularly involved in spatial aspects of perception. As such, their involvement in synesthesia is perhaps not surprising. Consistent with this, a recent fMRI study found activity in the posterior parietal cortex associated with number-space synesthesia.

Genes and Development

Synesthesia runs in families. Around 40% of synesthetes know of another first-degree relative with synesthesia, and there is little evidence for cultural transmission. Family members tend not to agree on the color of graphemes any more than unrelated synesthetes. Moreover, different types of synesthesia coexist within families. For example, synesthetes experiencing taste tend to have relatives with color synesthesia. This suggests that the gene has a restricted role in the determining the way that synesthesia manifests itself.

An examination of the variety of common induc-er-concurrent pairings suggests a possible mechanism. Vilayanur Ramachandran and Ed Hubbard note that one reason why graphemes and colors may tend to be paired is because of their anatomical proximity within the fusiform gyrus within the visual ventral stream. This adjacency principle extends to other types of synesthesia, such as number-space and lexical-gustatory synesthesia. The gene may promote localized connectivity within the brain and, depending on where it is expressed, it may give rise to different forms of synesthesia.

One debate that has yet to be resolved is whether or not synesthesia reflects extra feedforward connections between two regions of the brain or removal of inhibition on normally dormant pathways. A study using diffusion tensor imaging, which demonstrates more localized organization of white matter in synesthetes, is perhaps more consistent with the former view. However, other phenomena, such as acquired forms of synesthesia, are compatible with a disinhibited feedback account. Acquired forms of synesthesia can occur after ingestion of certain drugs (e.g., LSD) and also as a result of sensory loss such as blindness or even blindfolding. Given that these symptoms can occur within hours (after drug use) or days (blindness), it is more consistent with a release from inhibition. It is, of course, conceivable that different causal mechanisms in developmental and acquired cases could produce similar perceptual outcomes.

Why Does Synesthesia Exist?

The fact that synesthesia is relatively common and has a genetic basis (in nonacquired forms) leads to the question of whether it could be evolutionarily adaptive. From first principles it need not be so. There are many traits and conditions with genetic components that are disadvantageous or just benign. Although there is little evidence that synesthetes are at any net disadvantage, it could possibly be construed a benign variant of normal perception. This is effectively a null hypothesis that one would fall back on in the absence of any evidence for an advantage. One postulated advantage of synesthesia is that it leads to creativity. The claim is that more widespread differences in brain connectivity could facilitate novel and adaptive ideas. Under this account, the perceptual characteristics of synesthesia are not directly relevant to synesthesia's true adaptive function. However, the evidence is mixed (synesthetes are more likely to be artists, but artists don't necessarily score higher on formal measures of creativity). Another candidate is that synesthesia may enhance memory because stimuli (e.g., words) may additionally be stored perceptually (e.g., as colors). There is strong evidence that synesthesia does lead to improved memory. Under this account, the unusual perceptual experiences would have a direct impact on this adaptive skill, perhaps accounting for its common existence.